With development of electronic technology, miniaturization and multi-functionalization have become a trend of developing electronic devices. High energy secondary batteries, such as lithium-ion secondary batteries, are now widely used for providing electrical power to the electronic devices. Currently, the commercialized lithium-ion secondary batteries have high energy density and long cycle life, which can satisfy the most of the electronic devices. However, it is still desired to further improve the energy density, the cycle life and the safety of the lithium-ion secondary batteries so that the lithium-ion secondary batteries can satisfy more electronic devices such as electric vehicles.
A conventional lithium ion secondary battery consists of four main parts including a positive electrode, a negative electrode, a separator and an electrolyte. Generally, the cycle life and the safety of the lithium ion secondary battery are determined by the properties of the four main parts. In particular, a cathode material of the positive electrode will affect the performance of the lithium ion secondary battery as follows.
(1) During a charge-discharge cycle, electrons are migrated from lattices of the cathode material or into the lattices of the cathode material, thereby causing a volume contraction or a volume expansion of the cathode material. After a number of charge-discharge cycles, the lattices of the cathode material are partially damaged. Thus, the capacity of the lithium ion secondary battery is decreased.
(2) When the lithium ion secondary battery is in a charged state, transition metal ions in the cathode material are transformed into high valence metal ions with strong oxidability. However, the electrolyte is generally comprised of reductive organic compounds. The reductive organic compounds are oxidized by an oxidation-reduction reaction on the surface of the cathode material to generate water (H2O), carbon monoxide (CO), carbon dioxide (CO2) and other reaction products, which will cause the decrease of the capacity of the lithium ion secondary battery. Moreover, under a high temperature condition, the oxidation-reduction reaction of the reductive organic compounds can cause the combustion and the explosion of the lithium ion secondary battery.
(3) If the electrolyte includes a trace mount of active alcohols, the active alcohols can react with the cathode material in the charged state to generate H2O, CO, CO2. Thus, the capacity of the lithium ion secondary battery is also decreased.
To improve the properties of the cathode material, a coated cathode material is researched. In Chinese Patent No. CN 200310112600.9, Liu et al. disclose a cathode material coated with an oxide for a lithium ion battery. The oxide can be a metal oxide or non-metal oxide. The metal oxide can be an oxide of at least one of aluminum (Al), magnesium (Mg), zinc (Zn), calcium (Ca), strontium (Sr), lanthanum (La), cerium (Ce), vanadium (V), titanium (Ti), tin (Sn), and the non-metal oxide can be an oxide of at least one of silicon (Si) and boron (B). However, the metal oxide or the non-metal oxide has a high melting point, and is prone to forming its own crystalline phase. In other words, the metal oxide or the non-metal oxide can not form a compact film. Thus, the cathode material can not be effectively coated by the metal oxide or the non-metal oxide. Further, lithium ions can not selectively pass through the metal oxide or the non-metal oxide coating layer. Thus, in fact, the cathode material coated with the metal oxide or the non-metal oxide can not improve the charge-discharge performance and the cycle life of the lithium ion battery.
In Chinese Patent No. CN 200810063159.2, Wang et al. disclose a method for coating a cathode material with Li2O.2B2O3. The Li2O.2B2O3 is substantially similar to Li2B4O7 and Na2B4O7. It is well known that the Na2B4O7 is prone to absorbing water to generate Na2B4O7.10H2O. Similarly, the Li2O.2B2O3 is also prone to absorbing water. Therefore, it is very difficult to achieve dehydration during forming the positive electrode by using the cathode material with the Li2O.2B2O3. Moreover, the Li2O.2B2O3 can not form a compact film. Thus, the cathode material can not be effectively coated by the Li2O.2B2O3. Thus, in fact, the cathode material coated with the Li2O.2B2O3 can also not effectively improve the charge-discharge performance and the cycle life of the lithium ion battery.